The present invention relates generally to metallating ethylene polymers which have polymerized therein a minor amount of at least one nonconjugated diene. More specifically, the present invention relates to an improved process for metallating such polymers with a three-component metallation system. The present invention also relates to graft polymers and functionalized polymers prepared from the metallated polymers.
The term "metallation", used in its broadest sense, describes a substitution reaction in which a non-carbon atom is replaced by an alkali metal atom. In a narrower sense, it is believed that metallation of unsaturated polymers occurs by allyl hydrogen exchange whereby an anion is produced. The resulting anion is stabilized by delocalization with the adjacent double bond. The alkali metal atom is suitably provided by an organo-alkali metal compound.
Metallation of conjugated diene polymers with an organolithium compound, e.g., n-butyllithium, in combination with either a potassium alkoxide e.g., potassium-tert-butoxide or potassium-tert-amyloxide, or a tertiary diamine, e.g., tetramethylethylenediamine (hereinafter "TMEDA"), is known. See, A. F. Halasa et al., "Metallation of Unsaturated Polymers by Using Activated Organolithium Compounds and the Formation of Graft Polymers II", Journal of Polymer Science, Volume 14, pages 497-506 (1976). Halasa et al. note that each combination has one or more significant drawbacks. The n-butyllithium/TMEDA combination, while generally efficient for metallation of conjugated diene polymers such as polybutadiene, promotes polymer chain scission or degradation where there is a double bond in the polymer backbone. The n-butyllithium/potassium-tert-butoxide combination, when compared with the n-butyllithium/TMEDA combination, alleviates some of the chain scission problem but at a cost of lower metallation efficiency.
E. W. Duck et al., in U.S. Pat. No. 3,703,566, disclose metallation of unsaturated hydrocarbon elastomers and formation of graft copolymers by polymerizing one or more monomers in the presence of the metallated elastomer. The elastomer, more commonly known as an "EPDM terpolymer", has polymerized therein ethylene, a 1-olefin such as propylene and a nonconjugated diene monomer. Metallation is accomplished by admixing a solution of the terpolymer in an inert organic solvent with a complex of a saturated alkali metal hydrocarbon and a polar compound. The alkali metal hydrocarbon may be an alkyllithium compound such as n-butyllithium. The polar compound may be a tertiary diamine, such as tetramethylethylenediamine, or an alkali metal alkoxide, such as potassium-tert-butoxide. In other words, Duck et al., like Halasa et al., use a two-component metallation composition.
Polymer chain scission and low metallation efficiency are both undesirable. Polymer chain scission, or breaking up the polymer backbone, generally results in reduction of desirable polymer properties. Low metallation efficiency means excess metallating components may be present upon completion of metallation. Excess metallating components compete with the metallated ethylene polymer for reagents added subsequent to metallation. This competition provides a mixture of products rather than a generally pure product. Free alkali metal atoms (those not bound to the ethylene polymer) usually must be neutralized before use of the metallated polymer. The neutralized alkali metal atoms may adversely affect final product properties. Alkyllithium and potassium alkoxide compounds are also expensive. As such, final product cost escalates with increasing amounts of one or more of these compounds.
A metallating composition which provides enhanced metallation efficiency would be desirable. A metallated polymer with enough alkali metal sites to promote adequate functionalization or grafting of the polymer and thereby yield a useful polymer product would also be desirable. A metallated ethylene polymer suitable for use in preparing a thermoplastic elastomer would further be desirable. Graft polymers having either themoplastic or thermoplastic elastomer properties would similarly be desirable.